In susceptible individuals, certain Gram-negative bacteria such as Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumanii can cause serious infections, such as pneumonia, urinary tract infections, skin and skin structure infections such as wound infections, ear infections, eye infections, intra-abdominal infections, bacterial overgrowth in the gastrointestinal tract and bacteraemia/sepsis. The treatment of serious bacterial infections in clinical practice can be complicated by antibiotic resistance. Recent years have seen a rise in infections by Gram-negative bacteria which are resistant to many types of antimicrobials including broad spectrum antibiotics such as aminoglycosides, cephalosporins and even carbapenems. There is therefore a need to identify new antimicrobials that are effective against Gram-negative bacteria, in particular against multidrug resistant Gram-negative bacteria.
Polymyxins are a class of antibiotics produced by the Gram-positive bacterium Bacillus polymyxa. First identified in the late 1940s, polymyxins, particularly polymyxin B and polymyxin E (colistin, usually as its prodrug colistin methane sulphonate) were used in the treatment of Gram-negative infections. However, these antibiotics exhibited side effects such as neurotoxicity and nephrotoxicity. Nevertheless the polymyxins now play an important role in the therapy of MDR Gram-negative infections due to the lack of viable alternatives. However, their use in therapy is limited to treatment of last resort.
WO 2008/017734 tries to address this toxicity problem by providing polymyxin derivatives carrying at least two but no more than three positive charges. These compounds are said to be effective antibacterial agents with reduced renal toxicity. It is hypothesised in the disclosure that the reduced number of positive charges decreases the affinity of the compound for isolated rat kidney tissue which in turn may lead to a reduction in nephrotoxicity.
Certain des-fatty acyl polymyxin derivatives have also been disclosed with reduced acute toxicity in mice whilst retaining good activity against pseudomonads (Katsuma et al. Chem. Pharm. Bull. 2009; 57, 332-336; Sato et al. Chem. Pharm. Bull. 2011; 59, 597-602). The compounds were significantly less active than polymyxin B against E. coli and K. pneumoniae. 
WO 2010/075416 provides urea linked aryl polymyxin decapeptides including CB182,804, which is reported to have similar activity but reduced renal toxicity compared with polymyxin B. Phenyl cyclopropane polymyxin derivatives are also described in U.S. Pat. No. 8,415,307. These compounds are shown to have similar or reduced activity compared with polymyxin B.
WO 2012/168820 provides a further series of polymyxin derivatives reported to have reduced toxicity, and sometimes enhanced activity compared with polymyxin B, in which the diaminobutyrate group at position 3 in the tripeptide side chain is replaced by a diaminopropionate moiety.
Antibiotics are often used in combination for the treatment of infections for a number of reasons:                To broaden coverage of pathogens for empiric therapy or for treatment of mixed infections        To improve efficacy where the combination is more active than either antibiotic alone (additive) or more active than would be expected by simply summing the activity of the two antibiotics (synergistic)        To suppress resistance development        
Indeed polymyxins are sometimes used in combination with other antibiotics (such as rifampicin, carbapenems, aminoglycosides or quinolones) for treatment of serious infections in the clinic for all of these reasons. Numerous microbiological and animal efficacy studies have been carried out on polymyxin-antibiotic combinations (Petrosillo et al. Clin. Microbiol. Infect. 2008; 14, 816-827). Combinations of polymyxins e.g. with neomycin and bacitracin are also available for topical use. Polymyxins act on the outer membrane of Gram-negative bacteria and are believed to facilitate the uptake of antibiotics which are less capable to cross the outer membrane barrier and hence enhance their activity.
As well as the use of polymyxins per se in combination, it has been reported that des fattyacyl polymyxin derivatives such as polymyxin B nonapeptide (PMBN), although not having very potent antibacterial activity, are still able to enhance the activity of antibiotics whose uptake is hindered by the outer membrane (Vaara et al. Microbiol. Rev. 1992; 56, 395-411). PMBN has reduced acute toxicity compared with polymyxin itself though it is unclear as to whether renal toxicity is reduced.
The use of less toxic ‘permeabilisers’ in combination with a second antibiotic would seem to offer the potential for therapeutic preparations with potent activity and reduced toxicity.
Despite this approach having been considered for some years such preparations have not been brought into medical use because they do not offer sufficient improvements over available therapies. Notably, their activity often falls short of that of the analogous polymyxin-antibiotic combination.
The compounds of WO 2008/017734 have been tested in combination with rifampicin, clarithromycin and other antibiotics and show some synergistic activity.
WO 2009/098357 provides polymyxin derivatives having no more than three positive charges, such as described in WO2008/017734, but with short acyl chains. These derivatives have poor intrinsic antimicrobial activity but are capable of potentiating the activity of the other agents.
CB-182,804 in the presence of rifampicin shows MIC90 values equivalent or better than for polymyxin B plus rifampicin against E. coli and K. pneumoniae strains, but this compound is not quite as good against A. baumanii or P. aeruginosa strains (Quale et al. Microb. Drug Resist. 2012; 18, 132-136).
Activity of the desfatty acyl derivatives of Katsuma et al. and Sato et al. has not been reported in the presence of other antibiotics and neither have the compounds of WO 2012/168820 or U.S. Pat. No. 8,415,307.
There remains a need for less toxic polymyxin derivatives with strong potentiating activity for other antibiotics, and for combinations of such agents with partner antibiotics which offer therapeutic preparations with consistently potent activity across the target pathogens. Such compounds should also have an acceptable toxicity.
The present inventors have previously described in PCT/GB2012/052844, TW 101142961 and GCC 2012/22819, the contents of each of which are hereby incorporated in their entirety, polymyxin compounds for use in the treatment of microbial infections.
Surprisingly, the present inventors have found certain polymyxin derivatives, which have reduced toxicity compared to polymyxin or colistin, and are particularly effective at potentiating the activity of antibiotics such as rifampicin, and in some cases achieving enhanced in vitro potency compared to the polymyxin:antibiotic combination. Combinations containing these agents thus offer therapeutic options of consistently potent activity, but lower toxicity than currently available therapies.